Most passenger vehicles include some form of defogger or deicer arranged on the rear window or backlight. The defogger or deicer is implemented by placing a grid along the back window, typically horizontally, using conductive inks printed on the interior of the rear window. An electric current is passed through the grid to generate heat which sufficiently defogs or deices the window.
In order to insure production of a quality vehicle, automobile manufacturers typically test the grid to insure that each individual electrical conductor or line of the grid conducts current to insure proper operation. Because the grid is located on the interior of the window and manufacturers prefer to test the grid after installation of the window to the vehicle body, direct testing of the grid, such as by testing voltage across the end portions of each particular grid line requires assembly personnel to get into the back seat of the vehicle to conduct the test. Manufacturers prefer to avoid having the assembly personnel enter the vehicle to minimize the introduction of dust, debris, and dirt into the vehicle. Such testing can also be time consuming, as it requires that an operator enter the vehicle, conduct a test on each grid line, and exit the vehicle before proceeding to test the grid of the next vehicle having a heater grid. Further, such a process requires an operator strictly dedicated to performing this function.
One approach to making the testing process more efficient is discussed in U.S. Pat. No. 4,395,677, issued Jul. 26, 1993 and assigned to the assignee of the present invention, and which is hereby incorporated by reference. The approach discussed in this patent utilizes a pair of Hall effect sensors to test for current flow through each electrical conductor of the rear window heated grid in order to avoid having assembly personnel enter the vehicle on the assembly line. The solution produced by this system, requires two Hall effect sensors spaced apart a predetermined distance. This system, however, requires that the sensor be passed over the grid within a predetermined range of speeds, outside of which can produce inaccuracies within the system. Further, this system has been found to be sensitive to external magnetic fields introducing possibly inaccurate readings.
Thus, it is an object of the present invention to provide a tester for heated window grids which can be passed over the grid without regard to the speed at which it is passed over the grid.
It is a further object of the present invention to provide a more robust tester for heated window grids which is relatively unaffected by external electromagnetic fields.